With high integration and large capacity of a Large Scale Integration (LSI), a circuit dimension required for a semiconductor element becomes increasingly narrow. Using an original image pattern (that is, a mask or a reticle, hereinafter collectively referred to as a mask), a reduced-projection exposure apparatus called a stepper or a scanner exposes and transfers the pattern on a wafer to form a circuit, thereby producing the semiconductor element.
It is necessary to improve a production yield for costly LSI production. Furthermore, there is a demand for pattern formation having a line width of 10 nanometers in a contemporary typical logic device. At this point, a shape defect of the mask pattern and a deviation of various process conditions when the pattern is exposed and transferred on a wafer can be cited as a large factor that degrades the production yield. With a fine LSI pattern on the semiconductor wafer, the shape defect of the mask pattern becomes finer. Because dimensional accuracy of the mask is enhanced in order to absorb deviations of various process conditions, it is necessary to detect a defect of the extremely small pattern in the mask inspection. Therefore, high accuracy for inspection is required for an inspection apparatus that inspects the linewidth of a transfer mask used in LSI production.
There is a known inspection apparatus that inspects an inspection target mask by capturing an optical image of a pattern of the mask using an image sensor. In this inspection apparatus, the inspection target mask is irradiated with light emitted from a light source through an optical system. The mask is supported on a table and scanned with the emitted light by movement of the table. An image of the light transmitted through or reflected by the mask is formed on the image sensor through a lens, and the optical image captured by the image sensor is transmitted as measurement data to a comparison unit. In the comparison unit, the measurement data and reference data are compared to each other according to a predetermined inspection algorithm. A determination that a defect exists is made unless the measurement data matches the reference data (see for example, JP 2008-112178).
With the progress of microfabrication of the pattern formed on the mask, high magnification and high NA (Numerical Aperture) are being developed in an inspection optical system that captures the optical image of the pattern. Therefore, a focal depth that is of a permissible range of a distance between the optical system and the mask is deepened, focusing cannot be performed even by changing the distance between the optical system and the mask slightly, and the pattern image becomes blurred, which causes a problem in defect detection processing. For this reason, an auto focus unit that can maintain a focused state so as to always keep the distance between the optical system and the mask constant is used.
JP 2003-294420 discloses an auto focus unit that adjusts a focal position of an inspection optical system to a surface of the mask. In the auto focus unit, when the mask is irradiated with light from the light source, the light reflected by the mask is incident to an optical sensor. An electric signal of the incident light is digitally converted, and input to a height measurement circuit. The height measurement circuit outputs a difference signal between an input offset value and a target height. The difference signal is input to a Z-table driving circuit that drives a Z-table in a Z-direction (height direction). The Z-table driving circuit drives the Z-table in response to the difference signal. Therefore, the distance between the optical system and the mask can be kept constant to maintain the focused state.
In recent years, the pattern of the inspection target mask has become finer than before, and frequently a line width or a pitch of the pattern becomes less than or equal to a wavelength of the light used in the optical system of the inspection apparatus. When the line width or pitch of the pattern becomes less than or equal to the wavelength of the light used in the optical system for defect detection, diffracted light is generated in the pattern, and sometimes the focus position cannot correctly be detected. Particularly, in the case that the line width of the pattern of the inspection target mask becomes less than or equal to an optical resolution limit, since the pattern is not resolved, sometimes the focus cannot be put on the pattern. As a result, sometimes focus accuracy cannot be guaranteed in the sample inspection.
There is a known method, in which a focus measuring surface is provided in a predetermined place of the sample, the focus is put on the focus measuring surface, the focus is then put on the inspection position in the pattern of the mask based on the focus put on the focus measuring surface. However, during the inspection, a mechanical displacement of a position (Z-position) in a Z-direction (height direction), gravitational distortion of the sample, and a focus displacement caused by a change in temperature or atmospheric pressure are generated when the sample is moved in an X-direction or a Y-direction (horizontal direction). The mechanical displacement, the gravitational distortion of the sample, and the focus displacement have an influence on the inspection. Therefore, in using only the focus-measuring surface provided in the sample, sometimes the focus accuracy cannot be guaranteed in the actual inspection of the sample.
The present invention has been devised to solve the above problems. Considering the above, an object of the present invention is to provide a sample support apparatus that can accurately adjust the height (Z) position of the sample to the target position so as to be able to inspect the inspection position of the sample with high focus accuracy.
Other challenges and advantages of the present invention are apparent from the following description.